In building machines typified by power shovels and backhoes, the oil delivered from one hydraulic pump sometimes actuates plural actuators, such as motors for swing motion, left and right motors for travel motion, boom cylinders, arm cylinders, and bucket cylinders.
These actuators are connected with a hydraulic pump via their respective direction selector valves. The operation of the actuators is controlled by operating the direction selector valves separately. It is customary to regulate the flow rate of oil flowing into the actuators by pressure-compensating valves to prevent the working speeds of the actuators from changing if the load varies.
In the above-described building machines, plural actuators are often operated simultaneously. For example, a boom cylinder, an arm cylinder, and a bucket cylinder are driven at the same time. In this case, if the total oil flow rate required by the actuators working simultaneously is extremely large, then the capability of the hydraulic pump to deliver oil becomes insufficient, thus leading to a reduction in the supplied pressure. Then, the pressure-compensating valves fail to function normally. As a result, only the actuator on the side of the lighter load is driven; the actuator on the side of the higher load is not driven. The balance of working speed between the actuators is lost.
Techniques for alleviating these problems are disclosed in Japanese Utility Model Laid-Open No. 150201/1989, Japanese Patent Laid-Open Nos. 266302/1989 and 134402/1990. In these techniques, a shuttle valve is incorporated in each direction selector valve in addition to a pressure-compensating valve. Thus, a composite valve is formed. The pressure applied to each actuator by the load is detected and guided to both pressure-compensating valve and shuttle valve. Such composite valves are assembled into a unit to form a multiple valve. The greatest of the load pressures to the presently operating actuators is selected by the shuttle valve. The pump line pressure, or the pressure in the whole circuit, is adjusted by the selected greatest load pressure. The opening of the throttle of the pressure-compensating valve is controlled according to the difference between the greatest load pressure and the pump line pressure.
In the prior art techniques, the spool of the direction selector valve is equipped with a known throttling rod for switching the present set of actuator port and bridge port to the other set of actuator port and bridge port and for controlling the flow rate of the supplied pressure oil. In order to detect the load pressure in the actuator, the spool is provided with a hole forming a passage.
Specifically, a load-sensing port is formed in the center of the spool hole in the direction selector valve. The pressure-receiving surfaces of the pressure-compensating valve and of the shuttle valve face this load-sensing port. Dead-end load-sensing holes extend axially partially through the spool from its both side ends. Each load-sensing hole contains a land portion near its front end. When the spool is in its neutral position, the load-sensing port faces these land portions. Radially extending guide holes are formed in the land portions. The guide holes are called central guide holes. The load-sensing hole contains second land portions which are located between the actuator ports and their respective tank ports when the spool is in its neutral position. Other radially extending guide holes, called side guide holes, are formed in the second land portions.
In these prior art techniques, when the spool of the direction selector valve is in its neutral position, both central guide holes are located in the load-sensing port, and the side guide holes are placed in the tank ports. Thus, the load-sensing port is placed in communication with the tank ports. When the spool has shifted, the actuator ports are connected with the load-sensing port via the central guide holes and the side guide holes in the left or right load-sensing hole, depending on the direction of the shift. The load pressure to the actuator is made to act on the pressure-compensating valve in the direction to open it. At the same time, the pressure is sent to the shuttle valve.
These prior art techniques have the following disadvantages. When the spool of the direction selector valve begins to shift from its neutral position, the side guide holes contained in the load-sensing holes into which the load pressure is to be introduced are disconnected from the tank ports. Then, the central guide hole into which the load pressure is not admitted is disconnected from the load-sensing port. Under this condition, the load-sensing port is in communication with none of the tank ports. Thereafter, the side guide holes contained in the load-sensing hole into which the load pressure is to be introduced are placed in communication with the actuator ports before the throttling land portions start to permit the actuator ports to be connected with the bridge ports.
The relation of the area of the opening to the stroke of the spool is shown in FIG. 8. In particular, as soon as the stroke of the spool begins, the area of the opening permitting the fluid to flow from the load-sensing port LS to the tank ports T decreases down to zero. Then, the area of the opening permitting the fluid to pass from the actuator port B to the load-sensing port LS increases rapidly.
In the prior art techniques, therefore, when the spool reaches a certain point of stroke, the load pressure rushes into the load-sensing port, increasing the pressure inside this port rapidly. In this way, the pump line pressure rises steeply as shown in FIG. 9. This has posed problems.
Specifically, since the load-sensing port is also in communication with the entrances to the shuttle valve, the load pressure acts into the first entrance to the shuttle valve. At the same time, the load pressure from the other direction selector valve acts into the second entrance to the shuttle valve. The higher pressure is selected and delivered from the exit. Because this exit is in communication with the entrance to the shuttle valve of the next direction selector valve, it follows that the greatest load pressure is taken from the exit of the final-stage shuttle valve. This greatest load pressure is directed to an unloading pressure control valve connected with the pump delivery passage on the upstream side of the pressure-compensating valve.
When the spool of the direction selector valve is in its neutral position, the pressure inside the load-sensing port is not increased. Of course, the pressure inside the back pressure chamber in the unloading pressure control valve is low. The valve body of the unloading pressure control valve is opened. The oil delivered from the pump is returned to the tank under unloaded condition. When the spool of the direction selector valve shifts, the load pressure is detected to close the valve body of the unloading pressure control valve. This stops the oil delivered from the pump from being returned to the tank. The result is that the pump line pressure, or the circuit pressure, rises.
When the spool reaches a certain point of its stroke, all the load pressure pours into the load-sensing port via either load-sensing hole as mentioned above. This means that the greatest load pressure selected by the shuttle valve suddenly acts into the back pressure chamber in the unloading pressure control valve, closing the valve body of the unloading pressure control valve suddenly. As shown in FIG. 9, therefore, the pump line pressure increases at a high rate. The working oil under this increased pressure is forced into the actuator through the pressure-compensating valve, the bridge port, and the actuator port.
As a result, when the actuator of the prior art direction selector valve having load-sensing function is started to be actuated such as when travel motion is initiated or when the boom is started to be elevated, a large shock is produced, or large noise is created due to surging pressure. Another problem is that when the pressure in the actuator is increased gradually to lift the load slowly, it is difficult to control the pressure.